The chemical balance or elemental composition of a material directly determines the characteristics of that material's microstructure and subsequent properties and performance. The final elemental composition of a metal is generally determined by alloying and/or the primary process by which the material is produced. Therefore, it is imperative that the final elemental composition of materials, such as steel, be controlled accurately to ensure the consistency of its physical, mechanical, electrical, and magnetic properties and performance in service. The ability to monitor the in-process elemental composition of a liquid phase primary metal process, such as steelmaking, and subsequent alloying operations, would guarantee the chemistry, subsequent properties, and performance of the product.
In current metallurgical practice, the elemental analysis of the molten metal or alloy is accomplished by physically extracting from the melt a liquid sample which is quickly solidified and analyzed by either chemical or spectrometric means. If the appropriate chemistry is found to exist, the metallurgical process is terminated and the metal is poured. If the desired chemistry has not yet been obtained, the smelting or refining process is continued and the chemical analysis procedure is repeated. For example, for many basic oxygen steelmaking heats, the elemental analysis of one turndown sample is adequate to confirm that the steel has been made to specifications. However, when the chemistry is off and a reblow is required, another turndown sample must be taken and analyzed. Typically, an average of 1.5 samples are taken per heat, which represents up to 20 percent of the average 60 minute-cycle time required to refine a heat of steel. The delay or dead-time inherent to steelmaking as well as other metallurgical processes due to current elemental analysis procedures not only retards the productivity and efficiency of the process, but may also provide misleading results because the chemical activity of the process continues during the holding period required to conduct the analysis. A rapid in-process elemental analysis procedure would vastly improve the productivity, energy efficiency, quality, and economics of many metallurgical and other liquid phase processes and products.
Most past efforts to provide rapid, in-process, elemental analysis of molten metal systems were based on the use of an emission spectrometer, with spectral excitation data taken directly from a molten metal surface, or a plasma/ultraviolet spectrometer, analyzing powder produced from the molten metal. Efforts to perform spectrometric analysis directly from the surface of molten systems are described in U.S. Pat. Nos. 3,645,628; 3,659,944; 3,669,546; and 3,672,774. The inherent constraint to the practical application of this approach is the problem of proximity; that is, the stability and functionality of spectrometer equipment cannot be maintained in the immediate vicinity of an operating basic oxygen steelmaking furnace and many other metallurgical processes.
British efforts to provide spectral analysis based on metal powders generated from molten steel are outlined in U.S. Pat. No. 3,606,540. The problems with this technique include interruption of particle flow due to clogging of the lance and difficulty in positioning the jet tube. The lance clogging problem limits this probe method to a single analysis of three minutes or less duration. Once clogged, this lance must be replaced, which renders it neither cost nor time efficient. A variation on this technique (U.S. Pat. No. 3,602,595) generates metal powder by applying an arc to the surface of the molten metal. Spectral analysis of these arc-generated metal powders has also been found to be unreliable. While considerable laboratory work has been done on these and other techniques in the United Kingdom and in France during the 1960's and early 1970's, no technique has been reduced to routine practice in an industrial environment.